1. Field of Use
The present invention relates generally to electronic distance and position measuring systems and methods, and more particularly, to receivers providing electronic distance or position measuring capability where the distance or position is determined at the receiver by a comparison between the phase of a received pair of signals and of a reference signal of the same frequency.
2. Description of the Related Art
Many different types of distance or position measuring systems have been developed. Depending on the field application, the accuracy required, and the distance or position of the receiving station from the transmitting station(s), radio or optical (such as laser) techniques are utilized. Satellites are being used as relay or as transmitting stations in some systems.
The demand for improved systems is due to perceived public welfare, military, economic and other needs. For example, there is a public welfare need to be able to determine the location of in-flight aircraft. Radar and other electronic tracking systems are employed to provide such capability. Accurately locating surface and subsurface vessels in water bodies is necessary to prevent collisions and to provide navigation. Obviously, as the number of aircraft and vessels increase, the capability of such systems must increase.
There are many military applications for position or distance measuring systems. For example, the ability to determine the presence and location of "friendly" aircraft, land vehicles, and waterborne vessels must be available to effect desired coordination of them. Similarly, the ability to determine the presence and location of "enemy" aircraft, vehicles and waterborne vessels is equally important.
The dramatic rise in recent years of the price of certain natural resources has increased the need for accurate determination of the position of prospecting and mining vehicles and vessels. Position must be precisely determined because of the high cost of detecting and of mining such natural resources. Often, the vehicle or vessel cannot adequately determine or indicate its exact position.
It is often imprudent, if not impossible, for the prospecting or mining vehicle or vessel to mark its location for later reference. For example, in land prospecting or mining, it may not be prudent to leave location indicators. Further, prospecting often is performed from the air using aircraft and satellites; it is often difficult to coordinate the ground crew with the crew in the air or at a ground station. Land satellites also are subject to geological and physical changes. Deserts, for example, have been found to be abundant sources of certain natural resources such as oil and natural gas. Shifting of desert sand causes geological formations to change rapidly and causes location markers to be buried or moved.
Position in detecting problems in an undersea prospecting and mining environment often are more acute than on land. The ocean floor offers a significant opportunity for the recovery of specific natural resources (for example, oil, natural gas, metals, and rare elements). Locating marking buoys typically are not practical and may provide competitors with valuable information. Buoys usually can only be used in relatively shallow waters to mark an underwater location.
Deeper and deeper waters are being prospected to recover natural resources, such as oil and natural gas. State of the art technology now allows oil and gas wells to be drilled at depths previously impossible to reach. The cost of drilling such wells, however, is substantially greater than wells in shallower water. Further, leases of deep water drilling sites often are very expensive. Consequently, it is essential that the position of prospecting and of drilling sites be accurately determined. A well drilled at the wrong location can result in significant economic loss.
Often, for example, a well drilled 20 to 30 yards from a desired location produces signicantly less oil or gas than over drilled at the poper location; and the errant drill site may lie outside the lease hold. It can be appreciated that a prospecting or drilling vessel 200 kilometers offshore requires extremely precise location determining capability where an accuracy of +/-10 meters is required (in this example, the required accuracy would be 1/20,000).
Undersea oil and gas pipelines, communications cables, and electric power transmission systems are becoming more and more common. Often they traverse large expanses of water and are far removed from the nearest coastline. Accurate determination of their position is necessary to build and repair them.
The substantial and ever-growing need for distance and position measuring systems has spawned a variety of systems and methods. Some conventional systems employ optical means to determine the distance or position of the receiving station from a transmitting station(s). Laser beams often are used in such optical systems. The laser beam typically is focused by the transmitting station at the receiving station. The receiving station detects the laser beam and performs the required analysis to determine its precise position. Such an optical system can only be employed over short distances (typically up to 20-25 km). The optical frequencies at which the laser emits light often exhibit significant attenuation and/or distortion over the transmission path due to dust, smog, rain, mist, etc. The laser beam often has to be transmitted at a high power level to traverse the distance being measured. Any object in the path of the beam impairs, and may even prevent, proper operation. In certain situations, the high level of the laser beam particularly near the transmitting station can cause damage to an object in its path. The laser beam cannot curve with the surface of the earth. Often, tall towers are used at the transmitting and receiving stations to allow greater distances to be measured. Nevertheless, there is a maximum distance at which the transmitting stations can be displaced from the receiving station in order for such a system to be employed.
Satellites are employed in other conventional systems. Often, the satellite acts as a transmitting station that is positioned in the sky above the area in which the receiving station is located. The receiving station, typically using a microwave link, beams up data to the satellite and receives data back from the satellite used to determine distance or position. The satellite usually is displaced a substantial distance from the receiving station. As is well known, a satellite in a geostationary orbit (that is displaced from the earth's surface by distance of approximately 22,400 miles) rotates at the same rate as the earth so that it maintains its position constant relative to a given position on the earth's surface. Geostationery satellites are usually used in conventional system because the satellite needs to be located at all times so that distance or location can be determined. Such systems are susceptible to significant inaccuracies due to the very great distance between the satellite and the receiving station. For example, extreme accuracy must be achievable with a satellite in a geostationary orbit since it is displaced from the receiving station by at least about 22,400 miles. Satellite systems also are extremely expensive because the cost of constructing and launching a satellite is very high.
Other conventional systems transmit radio frequency signals (from the very low frequency bands up to the microwave bands) from the transmitting station to the receiving station. Typically, the transmitting station is located at or near the coastline. Various types of modulation techniques are employed. Some systems transmit data from the transmitting station to the receiving station, while others use the signals themselves to derive the desired distance or position information.
Within conventional radio frequency systems is a group that employs the strategy of measuring at the receiving station the difference in phase between the signal received from the transmitting station and a reference signal generated at the receiving station. The phase of the reference signal is or has been calibrated to be the same as the phase of a reference signal at the transmitting station used to generate the signal that is transmitted. Typically, very precise signal sources (such as atomic clocks) are used at the transmitting and at the receiving stations to generate the reference signals. Note that the precise signal source at the transmitting station usually is calibrated with respect to frequency and phase with the precise signal source at the receiving station prior to the system being put into service. Thereafter, over the time frame in which the calibration can be maintained, the reference signal generated by the signal source at the receiving station will have the same phase and frequency as the reference signal generated at the transmitting station.
As is well known, the phase of a signal changes in accordance with the distance of a receiving station from a transmitting station. To illustrate, imagine that a signal having a frequency of 1.8 MegaHertz (MHz) is generated at a transmitting station in accordance with a signal provided by an atomic clock and is transmitted to a receiving station displaced from the transmitting station. Because the signal has a frequency of 1.8 MHz, it must propagate only over a distance of approximately 166 meters in order for it to complete a full cycle.
Such conventional systems employ this fact in determining the position of the receiving station from the transmitting station. It can be appreciated that the phase of the received signal is directly proportional to the distance that the receiving station is displaced from the transmitting station. By generating a signal at the same frequency as the transmitted signal and having the phase equal to the phase of the signal source (atomic clock) at the transmitting station, the receiving station can determine its position by comparing the phase of this signal with the phase of the received signal.
Single signal systems do not provide the distance or position measuring capability that usually is required. Referring again to the example presented above, a signal having a frequency of 1.8 MHz goes through a complete cycle every 166 meters. After the receiving station has been displaced from the transmitting station by more than one complete cycle or wavelength (throughout the specification the term "lane" is used as another name for a complete cycle or wavelength; in the example presented here, a complete cycle, wavelength or lane is 166 meters), the receiving station cannot determine its position relative to the transmitting station unless the number of complete cycles or lanes can be counted or accumulated by the receiving station as it moves away from the transmitting station.
This counting of complete lanes is the approach used in conventional systems that employ a single signal phase measurement approach. The number of complete lanes typically is counted by the receiving station as it moves away from the transmitter station. The number of complete lanes is accumulated and used to provide the distance and/or location data.
The position of the receiving station is lost in such systems when the count of the number of complete lanes becomes inaccurate. This inaccuracy can be caused by several factors. Loss of the transmitting signal for a time period greater than the time required for the receiving station to travel more than a complete lane (in the example, this is 166 meters) results in an improper count of complete lanes. Loss of the transmitting signal can be caused, for example, by malfunctions in the transmitting or the receiving station, by interference or loss of signal due to propagation changes caused by changes in the ionosphere, or by interfering signals emitted by other transmitting stations, shipboard equipment, generating equipment, etc. Typically, the ionosphere exhibits radical changes over the course of a complete day at the frequencies that are employed in such a single signal system. During the daytime, for example, in the very low frequency band (1.5 to 2.0 MHz) only ground wave signals are propagated by the ionosphere and thus are received at the receiving station. (An illustration of this is the fact that only local stations can be received during daylight hours in the AM band in the U.S.) At night, however, the ionosphere becomes electrically charged, which results in reflective propagation. This results in multi-path problems at the receiving station (due to the reception of the reflected signal and the ground wave signal) or in loss of any received signal because the reflected signal hoos over and the ground wave signal does not reach the receiving station.
One typical approach used to provide a full lane count at the receiving station is to have the receiving station return to the transmitting station and then move back to the prospecting or drilling site. The high cost of operating a prospecting ship or drilling rig (often times several hundred thousand dollars a day), however, militates against having the ship or rig return to the transmitting site each time the lane count is lost. Another approach is to have a helicopter fly from the transmitting station to the receiving station to measure the number of complete lanes and to provide this lane count to the receiving station. The helicopter approach, which is the most commonly used one, is also extremely expensive because it requires the helicopter to fly from the transmitting station to the receiving station at least once a day. Note that when the lane count is lost, the ship or rig cannot do any work until the lane count has been acquired. This down time is caused because the exact distance or position of the receiving station cannot be determined when the lane count is lost. Thus, the helicopter must be immediately brought to the transmitting station and flown to the receiving station in order for the receiving station to begin work again. This essentially results in the dedication of a helicopter for the entire time that the ship or rig is in operation.
Related to conventional single frequency systems are systems that utilize a pair of signals transmitted from each transmitting station. The signal pair is used in an attempt to provide the ability to recover the lane count without having to return the receiving station to the transmitting station or to have a helicopter fly from the transmitting station to the receiving station to determine the lane count. Representative systems of this type are shown in the following U.S. Pat. Nos. 3,325,811 to Earp, 3,397,400 to Maass et al, 3,613,095 to Elwood, 3,797,015 to Elwood, 3,816,832 to Elwood, 3,839,719 to Elwood, 3,916,410 to Elwood, and 4,283,726 to Spence et al.
The technical strategy behind the conventional signal pair systems shown in these patents is that the pair of signals can be used to generate two measurement lanes: a fine lane and a coarse lane as they are typically called. As stated above, the term "lane" is synonymous with complete cycle or wavelength and means the distance at which a signal of a given frequency must propagate in order to go through a complete cycle. As is well known, the higher the frequency of a signal, the shorter its wavelength; the shorter the wavelength, the shorter the distance required for the signal to go through a complete cycle. Thus, for example, a signal having a frequency of 1.8 MHz goes through a complete cycle in 166 meters (which means the lane it defines is 166 meters long; in this example, this is the fine lane). In comparision, a signal having a frequency of 500 Hz goes through a complete cycle in approximately 600 kilometers (which means the lane it defines is 600 kilometers long; in this example, this is the coarse lane).
In these conventional two signal systems, the two signals are precisely displaced in frequency with respect to each other. Typically, the signal displacement is in the range of 500 Hz to 2 KHz. The close frequency spacing is due part to frequency spectrum restrictions imposed by licensing authorities in the countries where such systems are used. As is well known, the increased use of the frequency spectrum has resulted in severe crowding and deteriorated technical performance caused by such crowding. Licensing authorities usually will not allow a pair of signals to be displaced from each other by more than approximately 2 KHz.
In these two signal systems, the typical frequency displacement or difference between the pair of signals is from 0.5 to 4 KHz. For purposes of illustration, assume that the carrier frequency for each of these two signals is approximately 1.8 MHz and that they are displaced from each other by 1 KHz. One of the pair of signals is used to provide the fine lane measurement (which for a 1.8 MHz signal, as stated above, defines a fine lane which is 166 meters in length). The fine lane measurement is produced by comparing at the receiving station the received signal with a reference signal generated at the receiving station which has the same frequency (for example, 1.8 MHz), and a phase which is the same as the phase of the reference signal used to generate the transmitted signal at the transmitting station. The comparison of the phase of the received signal with the phase of the reference signal allows the position of the receiving station within a fine lane (which is 166 meters long) to be determined.
In such two signal systems, the total number or count of fine lanes is attempted to be determined at the receiving station by mixing the two received signals to produce a delta or difference frequency signal. The difference frequency signal has a frequency equal to the frequency displacement between the two received signals, and has the same phase that a signal having a frequency equal to the difference frequency would have if such a very low frequency signal was transmitted by the transmitting station and had propagated to the receiving station.
In the example, the two signals are displaced from each other by 500 Hz. The distance requirement for such a very low frequency 500 Hz signal to go through a complete cycle is 600 km. Instead, suppose such a 500 Hz signal could actually be transmitted by the transmitting station and received by the receiving station. The same coarse lane capability could be obtained. Practically, however, such a 500 Hz signal cannot be transmitted and received because it would require enormous antennas and transmitting and receiving equipment having very large sized tuned circuits. Further, it would require antennas and equipment entirely separate from that used to transmit and receive the 1.8 MHz signal that provides the fine lane.
The two signal system produces at the receiving station a low frequency difference signal having a very precise phase relationship to the transmitted signal, which difference signal can be used in an attempt to determine the count of fine lanes by which the receiving station is displaced from the transmitting station.
In such conventional systems, the phase of the difference frequency signal is compared with the phase of a reference signal generated at the receiving station having the same frequency as the difference signal, but whose phase is equal to the phase of the transmitting station reference signal that generates the pair of signals that are transmitted. The difference between the phase of the receiving station reference signal and the difference frequency signal provides an indication of the position in the coarse lane of the receiving station with respect to the transmitting station. In the example where the difference frequency signal has a frequency of 500 KHz, the receiving station can be removed from the transmitting station a distance up to 600 km before the difference frequency signal has gone through a complete cycle or lane. In this way, conventional systems attempted to determine the exact position of the receiver by first using the coarse lane measurement provided by the difference frequency signal to determine the count of the fine lanes that the receiving station was displaced from the transmitting station, and then zeroing in on the actual position within the fine lane by using the fine lane measurement.
In theory, such conventional systems appeared to be capable of providing total real time distance and position measuring capabilities. However, in practice, for the reasons discussed below, such real time distance and position measuring capabilities have not been obtained and such two signal systems have not achieved the commercial success that it was thought they would obtain.
The fine lane measurement in conventional two signal systems has a repeatable accuracy satisfactory to achieve a measurement of +/-10% of the fine lane length. The course lane measurement in conventional two signal systems also has a repeatable accuracy satisfactory to achieve a measurement of +/-10%, of the length of the coarse lane. The problem with such repeatable accuracy with respect to the coarse lane is that +/-10% of a course lane that has a 300 kilometer length is +/-30 km. Obviously, it is impossible with such an accuracy in the coarse lane to determine the fine lane count since each fine lane is only 166 meters in length. In other words, the coarse measurement capability in conventional systems is insufficient to determine the number of fine lanes that the receiving station is removed from the transmitting station.
Messrs. Spence and Martin in U.S. Pat. No. 4,283,726 addressed the course lane measurement problem from the perspective of the transmitting station and the transmitted pair of signals. Phase jitter and other phase error problems associated with conventional two signal systems employing single sideband and suppressed carrier generation techniques were substantially eliminated by the transmission of two continuous wave carrier signals displaced from each other by a preselected difference frequency amount. Spence and Martin found that the use of the pair of unmodulated carrier signals substantially improved the phase integrity of the transmitted signals as compared to the modulation techniques employed in prior systems. These improvements significantly reduced the phase inaccuracy caused by the transmitting side of the two signal distance and position measuring systems and methods.
However, despite the fact that the transmitted signals had been significantly improved with respect to phase integrity in the Spence et al. system, such a system still could not produce the desired accuracy on a repeatable basis with respect to the coarse lane measurement. Further, the accuracy of the fine lane measurement was not high enough in certain applications. For example, certain oil and gas leases require the location of the bore hole to be within one or two meters of a particular point.
The inventor has discovered that conventional receivers utilized in such conventional two signal systems contribute significantly to the inaccuracy associated with the coarse lane measurements and with the fine lane measurement. He has found that the propagation of the transmitted signals to the receiving station has also been found to introduce additional phase errors. He has found that conventional receivers introduce phase error during the processing of the received pair of signals.
Turning first to the phase errors produced by the propagation of the transmitted signal to the receiving station, there often are signals in addition to the ground wave signal which are received at the receiving station. The additional signals are due to the reflection of the transmitted signal from the ionosphere or from other paths. This creates what is called a multi-path signal situation, where the receiving station receives in very close succession more than one version of the transmitted signal. Because these signals travel paths of different lengths in their propagation from the transmitting to the receiving station, there is a phase difference between these various signals when they are received at the receiving station. This multi-path problem causes a measurement error to be produced both in the coarse lane and in the fine lane because the receiver is utilizing more than one version of the received signal(s) to produce the signals that are phase compared with the corresponding reference signals generated at the receiving station.
The multi-path problem typically has both a short term and a long term life span. With the respect to short term, it is often observed that the multi-path problem can be created for a very short amount of time (from several seconds to several minutes) and then go away. This is caused by such things as the position of the receiving station relative to the transmitting station, as well as the movement of other objects between the receiving station and the transmitting station. The multi-path problem can thus cause a distortion on a somewhat transient basis with respect to the fine measurement and the course measurement.
On the long term basis, the multi-path problem can occur during certain periods of the measurement day. As is well known, the characteristics of the ionosphere change, often dramatically, during a 24 hour period. Such change often is due largely to the fact that the ionosphere during sunlight does not reflect a transmitted signal (and thus the ground wave signal predominates), but after sundown reflects the transmitted signal and thus the multi-path problem. (This can be experienced by listening to the AM radio band in the United States at night when stations from great distances can be received that normally cannot be received during daylight.) In conventional systems, distance and position measurements cannot be obtained after sundown because of the severity of the multipath problem.
As stated above, the inventor has also found that conventional receivers contribute significantly to the unacceptable error exhibited by conventional systems. Phase distortion of the received pair of signals has been found to be caused by the signal processing performed by the receiver. Such phase distortion has not been addressed in conventional receivers because receivers typically are not utilized to produce a signal output having a precise phase relationship. Instead, conventional receivers typically have been directed to providing the ability to detect low level signals in a crowded frequency band so that the intelligence that they carry can be derived. Phase distortion typically is not a consideration in such receivers because the phase does not carry any intelligence that is needed.
One and two signal systems have highlighted the phase distortion problems associated with conventional receiver technology. Until these systems, there really was no need and also no ability to measure the phase distortion or error produced by conventional receivers. As stated above, there was no need in conventional receivers for such precise phase integrity because the phase did not carry any intelligence. Further, the measurement of the phase distortion was impossible to achieve in conventional receivers because there were no methods or equipment to measure it with a sufficient degree of accuracy.
One and two signal systems, however, have brought into focus the phase distortion deficiencies in conventional receivers.
In analyzing the phase distortions produced by conventional receivers used in two frequency signal systems, the inventor of the present invention has isolated several causes for these phase distortions caused by the receiver itself. Specifically, in addition to the multi-path signal propagation problem discussed above, the change in the received signal level (RSL) of each of the signals at the receiving station causes the receiver to introduce phase error or distortion to the received signals as they are processed by the receiver.
The inventor has also determined that phase distortions are caused by the variation in the temperature at which the receiver is operated. Phase errors are also caused by variations in the voltage level of the electrical sources which provide power to the receiver. All of these factors contribute to the generation of the phase distortions as the received signals are processed by the receiver.
The inventor has also determined that phase errors are caused by the bandpass filters utilized in conventional receivers. For example, the inventor has discovered that the bandpass filters, which are required to separate the received signals from each other and from adjacent signals in the frequency spectrum, produce phase errors because they do not exhibit a flat amplitude response over the pass band of the filter. Furthermore, conventional bandpass filters introduce a different phase distortion on the signals that are being filter depending on the level of the signal provided to the input of the filter. Reproducibility of a desired flat bandpass response is also unobtainable in conventional bandpass filters because of the variation in the phase change with respect to signal amplitude value exhibited by the crystal elements that are the integral components in such conventional bandpass filters.
Additional phase errors have been found by the inventor to be caused by variations in the required signal level (RSL) of the received signals. Conventional receivers are not designed to operate over the large dynamic RSL range that are experienced in such conventional two-signal systems. Specifically, as the receiving station gets further and further away from the transmitting station, the RSL of the received signal gradually decreases, so that at a certain distance, the signal is at a very low RSL level. Thus, the receiver at the receiving station must be able to provide appropriate signal processing of the received signals from a very high RSL (which is present when the receiving station is very near the transmitting station) to a very low RSL (when the receiving station is almost the maximum distance that it can be from the transmitting station) in order for the desired distance or coarse lane and fine lane measurement capability to be provided. Conventional receivers are not designed to operate over such a dynamic RSL range.
The inventor has also found that conventional receivers only generate one single difference frequency signal from the pair of signals that are received. Thus, conventional receivers only utilize a single coarse lane in performing the measurement needed to determine the fine lane count. Systems that produce more than a one difference signal employ a concomitant increase in the number of transmitted signals that are received. For example, two or three difference frequency signals could be produced in a system employing three transmitted signals, but conventional systems could not produce more than a one difference frequency signal if only two signals are being transmitted by the transmitting station.